Data stored in the ping-pong buffers by the ADC can be transferred to the internal Program Flash
Memory (PFM) in a two-step process. First, the contents of the ping-pong buffers are
transferred to buffer RAM using DMA. Next, the NVMUI is used to write the buffer RAM
contents to a designated page in the PFM. The DMA setup configurations are listed in
Table 3-3.
Table 1. DMA Setup Configuration
(DMA5)
| DMA5 Setup |
| SSA |
Source Address |
Beginning of ping buffer |
| SSZ |
Source Size |
512 (2 banks for ping and pong buffers) |
| SMR |
Source Memory Region |
GPR |
| SMODE |
Source Address Mode |
SPTR is incremented |
| DSA |
Destination Address |
Beginning of buffer RAM |
| DSZ |
Destination Size |
256 (1 bank for buffer RAM) |
| DMODE |
Destination Address Mode |
DPTR is incremented |
| SIRQEN |
Start Trigger Enable |
No (stops when DCNT reloads) |
| SIRQ |
Start Trigger |
N/A |
| SSTP |
Source Counter Reload Stop |
N/A |
| DSTP |
Destination Counter Reload Stop |
N/A |
| AIRQEN |
Abort Trigger Enable |
No (stops when DCNT reloads) |
| AIRQ |
Abort Trigger |
N/A |
How It Works:
- 1.Transfer the contents of the ping
buffer to buffer RAM by manually starting the DMA transfer. The DMA stops as
soon as the buffer RAM is full (DCNT reloads). Then, proceed to the next step to
write to PFM. In the next iteration (when the pong buffer is ready to be
transferred), simply start the DMA again manually. SPTR continues into the pong
buffer region, whereas DPTR rolls back into buffer RAM again. When the buffer
RAM is full and DMA stops again (DCNT reloads), the SPTR rolls back into the
ping buffer region(1).
- 2.The buffer RAM contents can be
written to PFM word-by-word at every sampling timer interrupt so that the CPU
stalls are minimized(2,3). The transfer is performed using the
write operation in the new NVMUI module in PIC18F57Q43.
- 3. Update write address location and number of pages written in the metadata.
Important:
- 1.In this application, the ping
and the pong buffers are adjacent to each other in data memory. If ping and
pong buffers are separated in the data memory, then SSA needs to be
reconfigured after every 256 iterations.
- 2.To write to PFM, word write
is used instead of page write because page write stalls the CPU for
typically 10 ms. This seriously affects the data sampling rate of ADC.
However, a word write typically takes about 50 µs, which can work with a 10
kHz ADC sampling rate.
- 3.PFM word writes stall the CPU
for typically 50 μs, which is about 50% time between two ADC samples (100 μs
sampling period). Therefore, an interrupt from the sampling timer is used to
ensure that PFM writes occur after the ADC has finished sampling and storing
data.